In the oil and gas industry, reservoir modeling involves the construction of a computer model of a petroleum reservoir for the purposes of improving estimation of reserves and making decisions regarding the development of the field. For example, geological models may be created to provide a static description of the reservoir prior to production. Reservoir simulation models may also be used to simulate the flow of fluids within the reservoir over its production lifetime.
One challenge with reservoir simulation models is the modeling of fractures within a reservoir, which requires a thorough understanding of matrix flow characteristics, fracture network connectivity and fracture-matrix interaction. Fractures can be described as open cracks or voids within the formation and can either be naturally occurring or artificially generated from a wellbore. The correct modeling of the fractures is important as the properties of fractures such as spatial distribution, aperture, length, height, conductivity, and connectivity significantly affect the flow of reservoir fluids to the well bore.
Mesh generation techniques are used in reservoir modeling. Two traditional mesh generation techniques for three-dimensional (3D) reservoir simulation are structured-based meshing and extrusion based meshing. In structured techniques, hexahedra are connected in a logical 3D i-j-k space with each interior mesh node being adjacent to 8 hexahedra. Extensions to structured techniques include local grid refinement where local regions of an original grid are replaced with finer grids. This can become time-consuming, computationally expensive, and prohibitively burdensome when dealing with general reservoir geometries, such as arbitrary 3D fracture surfaces. Because of the inherent 2.5 dimensional (2.5D) nature of existing extrusion techniques, similar limitations apply to these techniques. Alternative, fully unstructured meshing techniques exist, including tetrahedralization and polyhedral meshing schemes. The increased complexity of these techniques often leads to lower robustness as compared to structured techniques, especially, in the presence of imperfect geometry input.
Accordingly, simulation of reservoirs with large fracture systems, of arbitrary geometry and orientation, is difficult, with tradeoffs required between sufficient accuracy and reasonable computational time. As an example, simulation of a shale reservoir may generate a natural fracture network consisting of tens of thousands of fractures defined geometrically by hundreds of thousands of triangles. Such complex systems are not easily resolved using a 2.5-dimensional reservoir meshing system, and when resolved in three dimensions, often produce prohibitively large simulation models which take significant time to simulate.